In the field of optical communication, multi-channel communication is making rapid strides due to the advent of the wavelength division multiplexing (WDM) communication system. Along with this, optical elements are necessary in a quantity corresponding to the number of channels in order to achieve functional control over each channel. Examples of this are keeping the power of each channel consistent, and performing switching.
Accordingly, there is a growing need for small-sized optical circuit components that can be applied to optical switches and other such optical devices. A number of single-unit optical switches have been invented in the past, and matrix switches having a plurality of input/output ports, and in which a large number of these light switches are used, have also seen practical application.
Various techniques have been proposed for obtaining an optical switch. For instance, there is a method in which an input port and an output port are connected by mechanically moving them (see Patent Document 1, for example), a method in which an input port and an output port are connected by rotating a movable mirror to tilt it at a specific angle (see Patent Document 2 and Non-Patent Document 1, for example), a method in which liquid crystals are used (see Patent Document 3, for example), and a method in which the connection between an input port and an output port is changed by controlling the reflection of light by generating bubbles at the intersection point of connected waveguides or another such means. These are just a few of the various methods available.
Among these, a plan light wave circuit (PLC) type of device utilizing a thermo-optic phase shifter can be produced using semiconductor circuit production technology. Accordingly, the device easy to manufacture lends itself extremely well to integration, which is advantageous in terms of improving functionality and increasing scale.
A thermo-optic phase shifter is usually obtained as follows. First, an optical waveguide having a cladding layer and a core is produced on a substrate. A metal thin film or other such conductive thin film is formed on this optical waveguide and worked into a fine line shape along the optical waveguide, so that current can be conducted. When power is supplied to this thin film from the outside, heat is generated by the electric resistance of the thin film, so that the film operates as a heater of the optical waveguide. The heat generated by this heater reaches the core through the cladding layer of the optical waveguide. As a result, the refractive index increases in the portion of the optical waveguide that is heated by the heater. The effective waveguide length increases corresponding to the resulting change in the refractive index and to the waveguide length, and the phase of the light is shifted at the output terminal. The amount of phase shift can be controlled as needed by adjusting the power supplied to the heater. When the optical waveguide is formed from quartz glass, the refractive index temperature coefficient (dn/dT) of the quartz glass is about 1×10−5 (/° C.).
A light switch can be obtained by dividing a single optical waveguide into two optical waveguides at the input terminal, connecting at least one of the two optical waveguides to the thermo-optic phase shifter, and recombining the two optical waveguides at the output terminal. For example, if the phases of the light guided by the two optical waveguides are mutually shifted by one-half the wavelength, the output at the output terminal can be reduced to zero. Also, if the phases of the two divided optical waveguides are not shifted, the inputted light can be outputted without any modification. This allows on/off control of the output.
However, if a plurality of thermo-optic phase shifters are disposed in a single optical circuit for the sake of multiplexing, power consumption of the overall optical circuit is much higher when each thermo-optic phase shifter consumes a large amount of power. With the thermo-optic phase shifters that have been put to practical use up to now, such as when guiding light with a wavelength of 1550 nm (nanometer), which is normally used for optical communication, the power necessary to shift the phase by one-half the wavelength is about 400 mW (milliwatts) per channel. Therefore, if, for instance, an optical communication circuit with 40 channels is to be controlled, and a switch in which the above-mentioned thermo-optic phase shifter is utilized is provided for every channel, then a maximum power of 40×400 mW (that is, 16,000 mW, or 16 W) will be necessary. A method in which the heat generated by the heater is efficiently utilized has thus been proposed as a first proposal (see Patent Document 4, for example).
FIGS. 6 and 7 illustrate the conventional first proposal for efficiently utilizing heat generated by a heater. FIG. 6 is a cross section along the VI-VI line in FIG. 7. As shown in FIG. 6, with the thermo-optic phase shifter pertaining to this first proposal, there is a substrate 101 having a thickness of 0.8 mm and composed of silicon, for example. A sacrificial layer 102 is provided over this substrate 101. The sacrificial layer 102 is formed from phosphorus-added silica glass (PSG) obtained by doping glass with phosphorus, for example, and has a film thickness of 5 μm, for example.
A cladding layer 103 is provided over the sacrificial layer 102. The cladding layer 103 is constituted by a lower cladding layer 104 provided over the sacrificial layer 102, and an upper cladding layer 105 provided over this lower cladding layer 104. The lower cladding layer 104 and upper cladding layer 105 are formed from BPSG (boro-phospho-silicate glass) obtained by doping glass with boron and phosphorus, for example, and have a film thickness of 14 μm and 15 μm, respectively, for example. The substrate 101 may be formed from a semiconductor other than silicon, or from an insulator such as quartz glass. The sacrificial layer 102 is not limited to PSG, and may be formed from any material that has a higher etching rate than the substrate 101 and the cladding layer 103 and can be selectively etched with respect to the substrate 101 and the cladding layer 103, and as long as these conditions are met, may be formed from a semiconductor or a glass other such PSG, for example.
A core 106 that extends parallel to the surface of the substrate 101 is provided over the lower cladding layer 104, and the upper cladding layer 105 is provided so as to cover the core 106. The core 106 and the cladding layer 103 around the core 106 form an optical waveguide 107. The shape of a cross section of the core 106 perpendicular to its lengthwise direction is that of a rectangle with a height of 5.5 μm, and a width of 5.5 μm, for example. The core 106 is formed from a material with a higher refractive index than that of the cladding layer 103, such as GPSG (germanium-phosphorus-added silica glass), and the relative refractive index differential Δ between the core 106 and the cladding layer 103 is 0.65%, for example.
With the thermo-optic phase shifter of this first proposal, a thin-film heater 108 is provided over the optical waveguide 107, that is, on the surface of the upper cladding layer 105. The thin-film heater 108 is a thin film composed of chromium, and its thickness is 0.2 μm, for example. As shown in FIG. 7, the thin-film heater 108 includes electrode portions 108A at both ends, and a heater portion 108B in between the electrode portions 108A. The shape of the electrode portions 108A is square, for example, and the shape of the heater portion 108B is that of a slender wire with a width of 10 μm and a length of 4 mm, for example.
Of the region of the cladding layer 103 and the sacrificial layer 102 that are underneath the thin-film heater 108, grooves 109 extending parallel to the direction in which the core 106 extends are formed in regions located on both sides of the optical waveguide 107. The grooves 109 are formed at two places so as to flank the optical waveguide 107. The length of the grooves 109 in their lengthwise direction, that is, the direction in which the core 106 extends, is 4 mm, for example, the width of the grooves 109, that is, the length in a direction perpendicular to the direction in which the core 106 extends, is 205 μm, for example, and the depth of the grooves 109 is 29 μm, for example. The distance between the grooves 109, that is, the ridge width of the optical waveguide 107 is 25 μm, for example. The sacrificial layer 102 is removed from between the optical waveguide 107 and the substrate 101 to form a gap 111 (FIG. 6). The height of the gap 111 is equal to the film thickness of the sacrificial layer 102, and is 5 μm, for example. Consequently, the optical waveguide 107 is separated from cladding layer 103 other than the optical waveguide 107 and from the sacrificial layer 102 and the substrate 101 by the two grooves 109 and the gap 111, forming a bridge. The sacrificial layer 102 is formed over the entire surface of the substrate 101, except for the gap 111.
Thus, with the thermo-optic phase shifter of the first proposal, from the standpoint of preventing heat generated by the thin-film heater 108 from escaping to the substrate 101 side in order to reduce power consumption, the sacrificial layer 102 located underneath the optical waveguide 107 is removed, and the optical waveguide 107 is given a bridge structure.
FIGS. 8A to 8C schematically illustrate the method for manufacturing the thermo-optic phase shifter of the first proposal. First, phosphorus-added silica glass (PSG) is formed as the sacrificial layer 102 over the substrate 101 as shown in FIG. 8A, the lower cladding layer 104 is formed over this to dispose the core 106, and the upper cladding layer 105 is formed so as to cover this, to form an optical waveguide. The thin-film heater 108 is formed on the surface of the upper cladding layer 105.
Next, a resist 112 is formed over the thin-film heater 108 as shown in FIG. 8B, and this resist 112 is used as a mask to etch the grooves 109, which extend to the substrate 101 (composed of a silicon thin film), at locations flanking the optical waveguide.
Next, as shown in FIG. 8C, phosphorus-added silica glass (the sacrificial layer 102) is selectively removed by wet etching via the grooves 109 thus formed. Consequently, a thermo-optic phase shifter can be produced in which the sacrificial layer 102 does not remain, and the lower cladding layer 104 is disposed a distance away from and over the substrate 101, on the outside of the grooves 109.
Meanwhile, as a second proposal, there has been proposed a technique in which an optical waveguide is formed by forming overcladding so as to cover a core, a heater is formed over this optical waveguide, and grooves are formed to remove a silicon terrace (see Patent Document 5, for example).
FIGS. 9A to 9G schematically illustrate the method for manufacturing the thermo-optic phase shifter of the second proposal. First, as shown in FIG. 9A, a silicon thin film with a thickness of 2.5 μm, for example, is formed (not shown) by sputtering over the entire surface of a quartz substrate 121. This silicon thin film is patterned into a silicon terrace 122 by photolithography.
Next, as shown in FIG. 9B, undercladding 123 is formed in a thickness of approximately 8 μm by plasma CVD (Chemical Vapor Deposition). Sputtering is then performed to form a core film (not shown) with a thickness of approximately 6 μm, and to which germanium has been added, over the entire surface of the undercladding 123, and as shown in FIG. 9C, a core (optical circuit) 124 is formed by photolithography.
After this, as shown in FIG. 9D, overcladding 125 is formed in a thickness of 30 μm by flame deposition. Then, as shown in FIG. 9E, a heater 126 having three layers, namely, a titanium layer with a thickness of approximately 0.1 μm, a platinum layer with a thickness of approximately 0.5 μm, and a gold layer with a thickness of approximately 0.5 μm, is formed by the lift-off method. The gold is removed from the heat-generating region by etching, however, resulting in a two-layer structure of titanium and platinum.
Next, as shown in FIG. 9F, pits 127 are formed on both sides of the heater 126. The etching of these pits 127 is continued until they reach the silicon terrace 122. After this, as shown in FIG. 9G, the silicon terrace 122 is completely removed by etching, over the entire length in the lengthwise direction of the glass waveguide element. This product is then divided into individual elements by dicing, and irradiated with an excimer laser to form Bragg grating on the core 124, thereby obtaining a glass waveguide.
Since a gap can be formed between the optical waveguide and the substrate with these first and second proposals, the power consumption of the thermo-optic phase shifter can be reduced.    Patent Document 1: Japanese Unexamined Patent Publication No. H9-5653 (paragraph 0011, FIGS. 1 and 2)    Patent Document 2: Japanese Unexamined Patent Publication No. 2001-255474 (paragraph 0008, FIG. 2)    Patent Document 3: Japanese Unexamined Patent Publication No. S62-187826 (from page 5, lower-right block, line 4, to page 6, upper right block, line 14)    Patent Document 4: Japanese Unexamined Patent Publication No. 2004-37524 (paragraphs 0041 to 0044, paragraphs 0063 to 0065, and FIGS. 1 and 4)    Patent Document 5: Japanese Patent No. 3,152,182 (paragraphs 0024 to 0031, FIG. 2)    Non-Patent Document 1: Proceedings of the IEICE General Conference, C-3-8 (2002), p. 140